This invention relates to a polymer suitable for the preparation of liquid toners for developing electrostatic images and liquid toners incorporating such a polymer for developing electrostatic images.
A liquid toner formulation for the present invention generally comprises chemically modified polymer particles which may contain pigments or dyes as colorants, dispersed in a liquid carrier having an electrical resistance of at least 10.sup.9 ohm-cm and having a dielectric constant of not more than 3.5.
In general, such a liquid toner for developing electrostatic images is prepared by dispersing an inorganic or organic colorant such as iron oxide, carbon black, nigrosine, phthalocyanine blue, benzidine yellow, quinacridone pink and the like into a liquid vehicle which may contain dissolved or dispersed therein synthetic or naturally occurring polymers such as acrylics and their copolymers, alkyds, rosins, rosin esters, epoxies, polyvinyl acetate, styrene-butadiene, cyclised rubber, ethylene vinyl acetate copolymers, polyethylene, etc. Additionally, to impart or enhance an electrostatic charge on such dispersed particles, additives known as charge directors or charge control agents may be included. Such materials can be metallic soaps, fatty acids, lecithin, organic phosphorus compounds, succinimides, sulphosuccinates etc.
The object of the use of such liquid toners is to produce a visible image as an outcome of an electrostatic or electrophotographic imaging process. In some cases the final visual image needs to have certain mechanical properties such as high abrasion resistance. For this reason the solid thermoplastic epoxy polymers have been used in historical liquid toners. For these applications an epoxy resin is usually defined as the reaction product of bisphenol A and epichlorhydrin. Such compounds are often called diglycidyl ethers of bisphenol A, abbreviated to DGEBA. The structure of resins of this type is: ##STR1##
These materials have been shown to be useful in liquid toners because of their ease of comminution in non-solvents for epoxies, such as aliphatic hydrocarbons by either grinding or emulsification; their intrinsic positive polarity in such liquid carriers; and for the quality of images they produce regarding robustness, either as the unreacted thermoplastic polymer or after post imaging reaction using suitable crosslinking agents.
However, extensive work with these particular materials over the years has revealed certain shortcomings in the quality of liquid toners incorporating these polymers. In particular, stability of the dispersions containing these has been only fair due to the reactivity of DGEBA resins. Typical failure modes of these toners have been agglomeration of the dispersed epoxy, reduced redispersibility and loss of particle charge.
In order to overcome the limitations discovered, investigations were carried out to determine the probable source of this reactivity and discover a means of minimising it.
Epoxy resins, and in particular DGEBA as previously described, have long featured in liquid toner patents, for example U.S. Pat. No. 5,609,979 to Lawson. Advantage has been taken of their thermoplasticity for image fusing purposes, adhesiveness and high glass transition points for ease of particle size reduction. What has not been addressed before has been the instability of such materials in the presence of commonly used charge control agents having a slightly acidic nature.
It is well known in the surface coatings industry to modify epoxy resins by reacting with, for example, silicone resins to produce a silicone epoxy condensate or vegetable oils to produce epoxy esters. Such a silicone reaction is described by both Dow Corning and Shell Chemical in technical bulletins outlining uses of their products. In this case the reaction occurs between the hydroxyl groups pendant to the structure of the DGEBA molecule and the hydroxyl groups of the reacting silicone molecule. In the case of vegetable oils, it is the carboxylic acid group of the oil which reacts with the hydroxyl group of the DGEBA. The epoxy group terminal to DGEBA structure is not opened or altered by these reactions.
We have found in addition, and unexpectedly, that another class of materials will react with the epoxy resin in a similar way, i.e. at the hydroxyl groups, and result in a compound with very suitable liquid toner polymer attributes.
Thus an object of the invention is to provide a liquid toner containing a modified epoxy resin as the primary image forming polymer suitable for developing electrostatic images.
A further object of the present invention is to provide a liquid toner composition containing a modified epoxy resin having reduced reactivity towards metallic soaps or other toner constituents with an acidic nature.
Another object of the present invention is to provide a liquid toner composition having a modified epoxy polymer which retains its capability of crosslinking via the epoxide group contained within the DGEBA molecule.